logviewer tries to determine log directory via logback configuration

This introduces a new config variable, `logviewer.appender.name`, and modifies the logviewer process to locate the root log directory based on the the `FileAppender` named in the conf. Previously the logviewer assumed that the log directory was `{storm.home}/logs`.

If the named appender is not found or is not a `FileAppender` then the logviewer immediately throws a `RuntimeException`.

This issue is mentioned on the mailing list here: https://groups.google.com/d/msg/storm-user/IKRtIkqQfqc/OUDvyU6mxTMJ